As the number and variety of microelectronic and other low power devices continues to grow, so too the need for sources of power to run these devices. Wireless sensors, implantable devices, and other low-power gadgets typically obtain operational power in one of two ways: 1) by using electrochemical batteries or micro fuel cells and 2) by energy scavenging from environmental sources such as ambient heat, light, and vibration. Although electrochemical batteries and fuel cells can provide more power, they are not desirable for some applications due to their limited lifetime, size, and/or weight.
Energy scavenging is becoming more feasible because miniaturization and other technological advances have reduced power consumption. It is now possible to power at least some of these low power devices using harvested ambient energy in lieu of electrochemical sources such as batteries. Wireless microsystems and sensors have become so energy efficient that there are now viable designs that can scavenge sufficient operating energy from their surroundings. Vibration, or ambient kinetic energy, is one such source of ambient energy that abundant; for example, man-made machinery vibrates, trees sway in the wind, and of course humans produce an abundant amount of motion. However, most research and commercial efforts to develop vibration scavengers have focused around a single technological implementation: resonant generators. In other words, they are designed to harness energy coming in at a single steady frequency. These devices take advantage of an inherent mechanical amplification that occurs when the resonant frequency of the device is matched to the input vibration frequency. The use of resonance-based generators can have some drawbacks, however, such as the need to be tuned to their environment and the difficulty in scaling these devices when the vibration frequency decreases, both in terms of size and power density.